Elevated temperature working and heat treatment of stainless steel



Nov. 9, 1965 E s. NAGHTMAN 3,216,868

ELEVATED TEMPERATURE WORKING AND HEAT 7 TREATMENT OF STAINLESS STEEL Flled March 6. 1961 6 Sheets-Sheet 1 O 20 to 22 hour X 3 to 6 hours g to 1 hour A oinq Temperuture,F A zsoas FIGURE 1. EFFECT OF AGING TEMPERATURE ON THE HARDNESS OF SOLUTION-TREATED 17-4PH STAINLESS STEEL IN VEN TOR. fllioz .S', Nackimarz Nov. 9, 1965 E. S. NACHTMAN ELEVATED TEMPERATURE WORKING AND HEAT Filed March 6, 1961 TREATMENT OF STAINLESS STEEL 6 Sheets-Sheet 2 600 800 Aging Temperature, F

FIGURE 2 EFFECT OF AGE-'HARDENING TEMPERATURE ON THE I-LARDNESS 0F SOLUTION-TREATED 17-10 STAINLESS STEEL IN V EN TOR.

Z'llz'oi S. Nachzman 9, 1965 E. s. NACHTMAN ELEVATED TEMPERATURE WORKING AND HEAT TREATMENT OF STAINLESS STEEL 6 Sheets-Sheet 4 Filed March 6. 1961 howuu d 000m OOON COO- OOm 00- On 0- m mm E 6539.0 22mm 220895... 3 mE Q0 cu m com.

mm LOON.

6.59.852 059. 2.: 3 v0 30: 2 :3 00.6 c. 5 3 2 om m .52. 96 62:55 m 29. n 5 2 :3 \omn u 25 39:2 m uwz cozaom E0: 3555c Ubu 4 D00] a-ol Hdo p u s w/x INVENTOR. fllzoi $.Nach Tman I Nov. 9, 1965 E. s. NACHTMAN 3,216,868

ELEVATED TEMPERATURE WORKING AND HEAT TREATMENT OF STAINLESS STEEL Filed March 6. 1961 6 Sheets-Sheet 5 I 0 Room fem perclfure X o 600 F /aoo F X 800 F A 1000 F X IOOO F eo or tLLT Room Vickers Hordness,DPH,l0-kg load 6 Reduction In Areu,per cent FIGURE 5 EFFECT OF DRAFT ON HARDNESS OF 17-4PH STAIN- LESS STEEL HEATED AT INDICATED TEMPERA- TUBES FOR 3 TO 7 HOURS BEFORE DRAWING INVENTOR. Ellzot S. Nachzman aii'ys Nov. 9, 1965 E s. NACHTMAN 3,216,868

ELEVATED TEMPERATURE WORKING AND HEAT TREATMENT OF STAINLESS STEEL Filed March 6. 1961 6 Sheets-Sheet 6 I Room temperature 0 680 F u x 8 0 F Room 2 0 I000 F temperature 3 350 A I200 F |2OO F I V I400 F 1000 F .7 v I400 F 11. I 800 F O 3 300 l 600 F U II) J 250 l s Reduction in Areo,per cent FIGURE 6 EFFECT OF DRAFT ON HARDNESS OF 17-10P STAIN- LESS STEEL HEATED AT INDICATED TEMPERA- TURES FOR 3 TO 6-1/4 HOURS BEFORE DRAWING INVENTOR. Elliot .SNachiman YM [lit'ys United States Patent 6 3,216,868 ELEVATED TEMPERATURE WORKING AND HEAT TREATMENT OF STAINLESS STEEL Elliot S. Nachtman, Evanston, Ill., assignor to La Salle Steel Company, Hammond, Ind., a corporation of Delaware Filed Mar. 6, 1961, Ser. No. 93,668 Claims. (Cl. 14812) This application is a continuation-in-part of my application Serial No. 74,134, filed December 6, 1960, now abandoned, for Elevated Temperature Working and Heat Treatment of Stainless Steels.

This invention relates to a new steel having improved physical and mechanical properties, and it relates more particularly to stainless steels having these characteristics.

The products of this invention are characterized by a particularly high yield strength in combination with ade quate ductility. The process is capable of producing stainless steel having the above characteristics in combination with high strength. The materials of this invention has a high yield strength to tensile strength ratio, and the process for producing such products is characterized by great commercial acceptability in that treating times may be materially reduced.

The invention has been found equally applicable to steels of the austenitic and martensitic type. The process consists broadly of working the martensitic type at temperatures below the lower critical temperature, but above 250 F., or the so-called warm working range and working the austenitic types at temperatures above about 250 F. and below the upper limit of hot working temperatures for a given austenitic steel. Temperatures in the order of about 1700 F. would be considered a practical upper limit for the austenitic varieties. The process is particug larly applicable to solution-treated stainless steel of the austenitic or martensitic type which is capable of being precipitation hardened.

.It is an object of this invention to provide a stainless steel having high strength, high yield strength to ultimate strength ratio, and with good ductility.

It is an additional object of this invention to provide a process for the production of stainless steel having these characteristics.

It is an additional object of this invention to provide a process which will enable the production on a commercially acceptable basis of stainless steels of the austenitic and martensitic type having these characteristics.

These and other objects of this invention will hereinafter appear and for purposes of illustration, but not of limitation, particular embodiments are shown in the accompanying drawings, in which:

FIGURES 1 and 2 illustrate the effect of aging temperature on the hardness of solution-treated stainless steel;

FIGURES 3 and 4 illustrate the effect of the time of holding at the working temperature on the hardness of stainless steel; and

FIGURES 5 and 6 illustrate the eifect of the degree of working on the hardness of stainless steel.

. The recommended aging temperature of the manufac-' ice The essential process steps of this invention include solution treating at the temperatures generally recommended for the austenitic or martensitic stainless steel being used. This solution treating is followed by cooling directly to a preselected aging or working temperature or to room temperature. Ordinarily, quenching is preferred in order to avoid undesired precipitation where this would be a possibility. The aging or working temperature will lie between the lower critical and 250 F. for the martensitic type steels and between 250 F. and about 1700 F. for the austenitic type. It has been found that an optimum aging temperature exists for each different steel. This temperature is readily obtainable by running hardness tests on specimens aged at various temperatures.

.turer will, in many cases, prove to be the temperature ideally employed in this invention. If significant aging takes place, it is followed by working at the aging temperature. However, it has been found that in many cases excellent results can be achieved if the aging is eliminated, there being only a short stabilizing period prior to the working step. In any case, the working step is preferably carried out at the temperature which is found to be the ideal aging temperature, although different temperatureswithin the ranges cited are contemplated.

Superior results are obtained if a longer time for precipitation is allowed prior to the Working step. In this connection, it has been surprisingly found that the longer holding time prior to working, in addition to the working time itself, will be considerably less than the recommended aging time. This represents a substantial saving in heating requirements and furnace hold-up times.

Beneficial results can be obtained over a wide range of working percentages. Reductions from a few percent .to

50% and above are contemplated insofar as the broad aspects of this invention are concerned. 1

The novel concepts of this invention may be illustrated by the following examples, wherein reference is made to the following tables.

Element Type 17-10 Type 17-4PH Heat 26146 Heat 56418 Carbon I Manganese Phosphorus Nickel Copper Colnmhinm Tantalum Composition given in weight percent.

T able 2 .Room-temperature tensile properties of selected tainless steel specimens aged for short periods of time before drawing at elevated temperatures Aging Reduc- Vickers Y Stainless tion in Hard- Str., 2 Tensile Elong., Exten- Reduc- Specunen Steel Area by ness, ofiset Str., Percent sion tion in Grade Temp, Time, Warm DPH, 1,000 1,000 in Percent Area,

F. Min. Dwg., 10-kg. p.s.i. p.s.i. inch Percent Percent load 191 194 6 63 l7-4PH 800 415 189 190 (b) 68 4 g 76 107 8 8 17-1o1=' 1,200 5 10 s i 66 5 15 174PH. 800 5 10 34s 91 160 58 i 78 105 2 7 i-v 0 5 85 111 62 43 so original shoulder gage length original shoulder gage length b Specimen failed outside of punched gage marks.

1 Extension between shoulders;

u Sample quenched directly from the solution-treating furnace into the salt bath, aged for 5 minutes, and warm drawn through a die to reduce the cross-sectional area by 10 percent.

Table 3.R00m-temperature tensile properties of selected-stainless steel specimens Aging Reduc- Vickers Yield Strength, Stainless tion in Hard- 1,000 psi. Tensile Elongi, Reduc- Specimen Steel Area by ness, Str., percent tionin Grade Temp., Time, Warm DPH, 1,000 in Area,

F. Min. Dwg., 10,kg. 0.1% 0.2% p.s.l. inch percent percent load Offset Oflset 253 254 254 10 46 174PH 809 9 543 251 252 252 s 45 17-4PH 900 60 b 415 "fig 120 1 2 17-10? 1, 200 315 35 350 131 133 147 36 68 17-10P 1, 1, 448 b 286 e 68. 5 88 137 25 39 1,

Aged specimen 17-10P 1,200 1, 440 b 302 a 77. 5 98 5 143. 5 20 32 e Producer's typical properties.

b Brinell hardness.

u 0.02 percent offset.

These specimens were given a doubleaging treatment.

Table 4,.-,R00m-t emperatare, hardnesses of 17-4PH and 17-101 stalinless steels warm drawn after heating for sh rt times Room- Temperturc Vickers Hardness DPH, IO-kg. load Time at Tempera- Drawing ture Tempera- Before ture F. Drawing,

Min.

Nominal Reduction of Area,

Grade of Stainless Steel" Percent vrcnvrtncncnovcnmcncncamcncncncncncncnu wmvwmr wlor oam wlowcumwmrow I Solution treated at 1900 F. and quenched into salt bath heated at 800 F. Rods held at 800 F. for 5 minutes and then drawn as indicated.

?,Solution treatedat 2050 F. and quenched into salt bath heated at 1200 Rods held at 1200 F. for 5 minutes and then drawn as indicate EXAMP 1 ths inch diameter specimens made from the martensitic 17-4PH type stainless steel having the composition shown in Table 1 were solution heat-treated at 1900 F. for /2 hour and oil quenched. These specimens were then heated to about 800 F. in a salt bath and drawn through a .410 diameter die to efiect a reduction of 10%. The mechanical properties resulting from this treatment are shown in Table 2, reference being made to specimens 51 and 5-2. The exceptionally high yield strength to tensile strength ratio, the high strength and the relatively good ductility are of particular note. The 800, F. treating temperature was selected from the data shown in FIGURE 1. As can be seen, the hardness curve obtained after the various aging temperatures provides an optimum aging temperature.

EXAMPLE 2 V ths inch specimens of an austenitic 17-1 0P stainless steel having a composition as shown in Table 1 were solution heat-treated at 2050 F. for /2 hour. The specimens were water quenched and heated to about 1200 F. in a salt bath. They were then drawn through a .410 diameter die to efiect a reductionof 10%,. The results are shown in Table 2, reference being made to specimens 6-1 and 6-2. It is noted that the yield strength was raised in an extremely short time. The specimens also are characterized by excellent ductility. The treating temperature of 1200 F. was selected from the curve shown in FIGURE 2. As is suggested by the curve, there is a likelihood that a higher aging time will produce a greater hardness. This is in line with manufacturers data, which recommends temperatures above 1200 F. for optimum aging.

EXAMPLE 3 To illustrate the exceptionally high strength that can be accomplished by this process, a series of hardness tests. were performed, the results of which are shown in FIG.- URE 3. The composition of the, 17- 4PH specimens was the same as that employed in Example 1. The specimens having the maximum hardness were selected and tensile tests were carried out on these specimens, the results of which are shown in Table 3. The tests were performed on specimens aged at 800 F. for approximately 3-7 hours, and reduced 35% in cross-section. The mechanical testing reveals an exceptionally high yield strength to tensile strength ratio, very high ultimate strength, and adequate ductility.

EXAMPLE 4 In order to illustrate the time-saving value of the instant process while still achieving maximum hardness, specimens of the 17-10P variety having the same composition as those of Example 2 were tested. Hardness tests, the results of which are shown in FIGURE 4, reveal that a 1200 F. treating time and 35% reduction in crosssection will produce a high strength specimen. It is noted that FIGURE 4 reveals that the maximum hardness is achieved in approximately 5 hours. This is in contrast to the manufacturers suggested aging times of 24- 36 hours. The tensile tests reveal high tensile strength, high yield to ultimate ratio, and good ductility (see Table 3). It is immediately apparent that the producers typical properties do not approach those achieved by this process.

EXAMPLE 5 In order to illustrate the effect of quenching on the properties of the austenitic and martensitic varieties, specimens were made up having the compositions of Examples 1 and 2. The 17-4PH and 17-10P specimens were treated as shown in Table 4. All the specimens were quenched to room temperature except those marked (a) and (b). The results show that with the martensitic variety, there is a substantial difference when not quenching directly to room temperature. It is equally apparent that there is substantially no effect on the specimens of the austenitic variety. Thus, with a martensitic type stainless steel, it becomes necessary to transform to the martensitic state before the working operation in order to achieve the desired properties.

The data shown in FIGURES 5 and 6 illustrates that higher strength is achieved as higher working drafts are resorted to. There is, of course, a consequent sacrifice in ductility to be expected. However, it is contemplated that drafts in the neighborhood of 50% and above could be employed.

Although the invention has been described with reference to particular steels of the austenitic and martensitic variety, the invention is intended to apply to all stainless steels, including the 300 and 400 types, and other special types such as the so-called sub-zero varieties. In the latter case, as is the case with all the martensitic types, it is necessary to transform prior to the working step. As should be obvious from the above examples, the invention is particularly advantageous when applied to steels of the precipitation-hardening type, and all stainless steels of this nature are contemplated. In this connection, the elevated temperature drawing has been shown to enhance and accelerate the aging phenomena.

It will be understood that changes may be made in the above process in a manner to provide the characteristics of this invention without departing from the spirit thereof, especially as defined in the following claims.

I claim:

1. The process of treating stainless steel hardenable by some mode of precipitation comprising the steps of solution treating the steel, cooling the steel and introducing the steel into a medium maintained at a temperature above 250 F., holding the steel at this temperature for a time suflicient to produce a substantial hardness increase in the metal without over-aging the metal, and finally working the steel while it is still maintained at this temperature to achieve a reduction of a few percent up to about 50 percent.

2. The process according to claim 1, wherein the steel is an austenitic stainless steel and the cooling consists of quenching to room temperature.

3. The process according to claim 1, wherein the steel is an austenitic stainless steel and the cooling consists of quenching directly into said medium.

4. The process according to claim 1, wherein the steel is a martensitic stainless steel and the cooling consists of quenching to transform the steel to martensite.

5. The process .of treating stainless steel hardenable by some mode of precipitation comprising the steps of solution-treating the steel, quenching the steel to maintain a supersaturated solid solution, reheating the steel at an elevated temperature above 250 F. and at a temperature wherein precipitation from solid solution will begin, and working the steel at elevated temperature during which precipitation will continue whereby precipitation will occur during working at the elevated temperature.

References Cited by the Examiner UNITED STATES PATENTS 2,365,956 12/44 Hodge et a1. 148l2.4 2,369,805 2/45 Schultz 148-135 2,527,521 10/50 Bloom 148-135 2,801,916 8/57 Harris et a1 148-38 2,892,702 6/59 Walton et a1 148-136 2,905,577 9/59 Harris et a1 148-12 2,934,463 4/60 Schmatz et al 148-12 3,053,703 9/62 Breyer 148-12 DAVID L. RECK, Primary Examiner.

MARCUS U. LYONS, RAY K. WINDHAM, ROGER L. CAMPBELL, WINSTON A. DOUGLAS,

Examiners. 

1. THE PROCESS OF TREATING STAINLESS STEEL HARDENABLE BY SOME MODE OF PRECIPITATION COMPRISING THE STEPS OF SOLUTION TREATING THE STEEL, COOLING THE STEEL AND INTRODUCING THE STEEL INTO A MEDIUM MAINTAINED AT A TEMPERATURE ABOVE 250*F., HOLDING THE STEEL AT THIS TEMPERATURE FOR A TIME SUFFICIENT TO PRODUCE A SUBSTANTIAL HARDNESS INCREASE IN THE METAL WITHOUT OVER-AGING THE METAL, AND FINALLY WORKING THE STEEL WHILE IT IS STILL MAINTAINED AT THIS TEMPERATURE TO ACHIEVE A REDUCTION OF A FEW PERCENT UP TO ABOUT 50 PRECENT. 